Injection molded optical disks have grown enormously in the last five years worldwide. In the U.S. alone is 1990, over 400,000,000 audio compact disks (CD) were made and sold. This does not include the millions of video disks or the various optical data storage computer memory disks such as CD-ROM or erasable magneto optical disks. As is typical with a high production-volume manufacturing operation, productivity has also greatly increased in the last five years, with the injection molding cycle time of CDs going from "highest state of art" five years ago at twelve seconds or more, to less than half that today. Still, there remains considerable room for improvement, as can be seen by considering that each injection molding cycle consists of three stages:
1. Injection fill and pack--during which time the molten thermoplastic is substantially flowing.
2. Cooling and solidification time--after meltflow has substantially ceased, the molded optical disk must be sufficiently cooled below its solidification point and glass transition temperature so as to be able to withstand the mechanical shape-distortion forces occurring during ejection; also, the molten sprue must be at least substantially solidified so that it will release from the mold and will not excessively slump or string. (Applicants' U.S. Pat. No. 4,793,953 patent issued Dec. 27, 1988 and U.S. Pat. No. 5,068,065 patent issued Nov. 26, 1991 address these problems, respectively.)
3. Disk removal--the "mold open" time is defined as starting from the time when the clamping forces exerted on the mold are released and the molded disk may be withdrawn from the open mold, and ending only after the mold is again closed and sufficient clamp force has been applied to permit injection to start a new cycle.
In addition, the centerhole must be formed in the optical disk and the sprue must be removed sometime in the cycle.
All known commercially-available injection molding processes for optical disks today (other than Applicants') employ the steps of:
1. opening the disk moldset by axial travel (along the X axis in X, Y, Z coordinate space, for a horizontal injection molding machine configuration used commonly for CD molding) of the movable platen "rearward" (away from the stationary platen side) at the plane of the parting line (designated by Y and Z coordinates, for the horizontal injection molding machine configuration)
2. swinging into position a robotic arm (traveling first in the Y, Z plane, then "stripping in" along the X axis,) until vacuum actuated suction cups mounted at the end of the arm compress onto the disk surfaces near the centerhole (the sprue has already been removed from the molded disk by some punching operation in these "prior art" processes)
3. applying the vacuum with sufficient force to overcome whatever disk retention forces may exist to hold the disk onto the molding surface,
4. reversing the motions of the robotic arm (stripping out along the X axis then swinging out in the Y,Z plane again, to permit the disk moldset to be again closed (by the step of "forward" axial travel of the movable platen toward the stationary platen side) and clamped for the next injection cycle.
Over the last five years, a number of improvements have been made to progressively reduce the amount of time these disk removal steps require. In general, reducing the mass to be moved will make these operations faster, as will also reducing the distance to be traveled. Overlapping simultaneously/concurrently any timed sequences which do not have to be done "in series" or sequentially will be faster. For example, the robotic arm can start to swing into place even while the mold is still opening. Reducing the mass and thickness of the arm allows for use of a smaller mold opening strokelength and faster acceleration and deceleration, provided rigidity is also retained. In addition, the robot arm pivot can be mounted directly onto the moveable side of the moldset, so its proximity and distance to travel is shorter. These improvements have reduced the disk removal time from 3-4 seconds to 1.5-2 seconds within the last five years. However, this still comprises approximately 25-33 percent of the total injection molding cycle time for a CD today. The use of vacuum actuated suction cups in particular costs cycle time, since each actuation time is approximately 0.15 second, and valve actuation and signal delays are inherent to such a system. Its inherent complexity causes a lack of durability or robustness for high production operations, resulting in downtime.